Description will be made of the background art by way of a hybrid shovel as an example.
In a hybrid shovel, from the perspective of protecting a power storage device, a charging rate that does not exceed an “upper-limit working voltage” of the power storage device even when supposed maximum regeneration energy (maximum regeneration energy) generated by a revolving operation or the like is absorbed is set in advance as an “upper-limit working charging rate”. Moreover, the hybrid shovel controls the power storage device so that the upper limit of the charging rate of the power storage device is determined so as not to exceed the upper-limit working charging rate (see Japanese Unexamined Patent Publication No. 2010-41828).
The “power storage device” in the present specification includes both a secondary battery (a so-called “battery”) such as a lithium-ion battery and a capacitor (an electric double-layer capacitor). However, for the sake of convenience, in the embodiment of the present invention, the power storage device will be referred to as a “battery”.
Moreover, the “upper-limit working voltage” refers to a battery voltage (that is, an upper limit of a practical battery voltage) at which the hybrid shovel stops due to a battery abnormality if the voltage exceeds the upper-limit working voltage.
Here, when a battery deteriorates with use as well as aging, an internal resistance thereof increases and the battery voltage increases. Thus, an overvoltage state may occur.
On the other hand, it is technically difficult to detect a deterioration of a battery appearing as an increase in the internal resistance directly.
As a countermeasure to this problem, in the related art disclosed in Japanese Unexamined Patent Publication No. 2010-41828, an upper-limit working charging rate is set to a value in which a margin corresponding to a certain deterioration amount is taken into account by predicting a future deterioration. That is, in Japanese Unexamined Patent Publication No. 2010-41828, the upper-limit working charging rate is set to such a low value that the battery voltage does not exceed an upper-limit working voltage when regeneration energy is absorbed even if the battery deteriorates.
However, the deterioration progresses with use, and for example, the degree of deterioration of the battery is different depending on whether the battery is a new product or has been used for a long period. However, in Japanese Unexamined Patent Publication No. 2010-41828, the upper-limit working charging rate is set to a low value by taking a margin corresponding to a certain deterioration amount into account regardless of the degree of deterioration of the battery and is not updated. Due to this, the upper-limit working charging rate of a new battery is too low to provide an original battery performance. That is, since the upper-limit working charging rate is lower than an upper-limit working charging rate that is to be set originally, the remaining capacity of the battery is not used sufficiently. Further, the upper-limit working charging rate of a battery used for a long period is too large that an overvoltage state in which the battery voltage exceeds the upper-limit working voltage may occur during absorption of regeneration energy.
On the other hand, Japanese Unexamined Patent Publication No. 2013-119761 discloses a technique of detecting an accumulated operating time of a machine or a battery voltage serving as an indicator of the degree of progress of a battery deterioration and specifying a correction value corresponding to the detected accumulated operating time of the machine or the detected battery voltage from a correction value map to thereby correct an upper-limit working charging rate using the specified correction value.
However, the accumulated operating time and the battery voltage are merely indicators of deterioration (that is, roughly estimated signs) and do not always indicate the degree of deterioration directly.
Moreover, the state of progress of battery deterioration is not constant but is different depending on the use condition (for example, a temperature environment), the use frequency, and the like.
Thus, in the related art disclosed in Japanese Unexamined Patent Publication No. 2013-119761, it is impossible to detect the degree of deterioration of a battery exactly and perform precise charging control by reflecting the degree of deterioration.